1. Field of the Invention
The present invention relates to a hard coating film having a hard coating layer stacked on a transparent plastic substrate film which is free of internal haze and has microscopic pits formed in a specific shape with a specific frequency in the surface, thereby inhibiting occurrence of interference unevenness traceable to reflection between the hard coating film and the film substrate, and a manufacturing method thereof, and further to an antireflective film, a polarizing plate and an image display unit each having an antireflective layer stacked on such a hard coating film.
2. Background Art
An antireflective film is generally placed on the display surface in image display units various in their types, such as a liquid crystal display (LCD), a plasma display panel (PDP), an electroluminescent display (ELD) and a cathode-ray tube display (CRT), for the purpose of preventing a contrast drop from occurring by reflection of outside light from the display surface and surroundings' reflection in the image display unit. Therefore, the antireflective film is required to have not only high antireflection capabilities but also high transmittance and high physical strength (e.g. high resistance to scratching).
An antireflective layer for use in the antireflective film has so far been formed of a single-layer or multilayer thin film. In the case of a single-layer film, it is essential only that a layer having a lower refractive index than a substrate film (a low refractive-index layer) be formed in a thickness equal to a quarter of the design wavelength in optical-thickness terms. When further reduction in reflection is required, formation of a layer having a higher refractive index than a substrate film (a high refractive-index layer) between a substrate film and a low refractive index layer is known to be fine.
Such an antireflective film is suited for mass production and cost reduction, and therefore the antireflective film formation in a Roll-to-Roll mode utilizing a combined method of wet coating and UV cure has prevailed in recent years. With the growing demand for antireflective films formed through the use of such a wet coating method, it has been intensely desired to develop antireflective films which allow further reduction in costs and further improvement in functionality while keeping up low costs.
In the case of forming an antireflective film by using a transparent plastic film as a substrate film, a makeup that, after a hard coating layer having a thickness of the order of 1 to 20 μm is formed on the substrate film, an antireflective layer is formed thereon is widely adopted for the purpose of supplementing physical strength of the plastic film. Herein, when there is a difference in refractive index between the hard coating layer and the substrate film, reflection occurs at the interface between the hard coating layer and the substrate film, and causes interference with light reflected off the hard coating layer surface; as a result, interference unevenness develops (see JP-A-2005-14479 for the mechanism of occurrence of interference unevenness).
Another method for enhancing antireflection capabilities is also known in which two or more antireflective layers including a high reflective-index layer and a low reflective-index layer as mentioned above are stacked (see e.g. JP-A-11-153703). However, such a method involves an increase in number of layers and requires repeating the procedure including coating, drying and curing steps at least three times, and thereby a serious drop in productivity is caused. In addition, our study has revealed that the higher antireflection capabilities and the lower refractive index an antireflective film has, the more conspicuous the antireflective film for its interference unevenness.
As a method of increasing antireflection capabilities without increasing the number of antireflective layers in contrast to the above method, still another method is known in which, in a structure of having on a hard coating layer one low refractive-index layer as an antireflective layer, a high refractive index is imparted to the hard coating layer, and thereby the effect of interference between the hard coating layer and the low refractive-index layer is heightened to result in enhancement of antireflection effect (see e.g. JP-A-7-287102).
However, such a method has a problem that interference unevenness tends to become obtrusive because of a great difference in refractive index between the hard coating layer and a substrate film.
As a method for solving such a problem, there is a known method in which a solvent capable of penetrating into a substrate film is used in a coating composition for formation of a hard coating layer, thereby reducing interference unevenness (see e.g. JP-A-2007-84815). However, such a method introduces severe restrictions on usable solvents and substrate films, and besides, when the time for drying a hard coating layer is reduced with the intention of increasing productivity, effects of solvent's penetration into a substrate cannot be produced to a sufficient degree, which results in failure to bring about the effect of preventing interference unevenness. In addition, the method is difficult to control when there is a great difference in refractive index between the hard coating layer and the substrate film.
With the intention of solving those problems, JP-A-2004-345333 has advanced a proposal to provide an interference-unevenness prevention layer specific in refractive index and thickness between a substrate film and a hard coating layer. According to this proposal, neither solvent to be used for stacking a hard coating layer nor drying time is subjected to restrictions, and the prevention layer provided is effective even when there is a great difference in refractive index between the hard coating layer and the substrate film. In other words, the proposal can solve all the concerns. However, it involves the provision of a new layer, and has a problem in terms of productivity.
Based on a concept different from the above, the method of providing projections between a substrate and a hard coating layer and thereby preventing interference unevenness has also been suggested (see e.g. JP-A-2005-14479).
Antireflective films are broadly divided into two types, namely films of the type which utilize a method of scattering reflected light by providing asperities on the surfaces and thereby preventing outside images from being reflected therein (antiglare films), and films of the type which utilize a method of stacking antireflective layers without making any modification to the surface smoothness to lower their reflectivity, thereby preventing outside images from being reflected therein (clear-type antireflective films). Of these films, films developing interference unevenness traceable to differences in refractive index between their respective substrate films and hard coating layers are antireflective films of the type which have smooth surfaces. A reason for venturing to use clear-type antireflective films even though they tend to develop interference unevenness consists in that these films cause no scattering, and consequently the image display unit equipped with them is free of blurring of images (referred to as blurring of characters too).
Provision of projections between a substrate film and a hard coating layer causes scattering, a rise in haze and blurring of images resulting from the scattering. Thus it is undesirable to bring about elevations of haze in clear-type antireflective films.
In summary, it has been desired to develop hard coating films which each are free of interference unevenness caused by reflection from the interface between each individual transparent substrate film and hard coating layer, notably in cases where large differences in refractive index are present between individual substrate films and hard coating layers, ensure high productivity, have low haze and cause no blurring of images, antireflective hard coating films made by stacking antireflective layers on hard coating films having the foregoing features, transparent substrate films for use in hard coating films having the foregoing features, and manufacturing methods of those films.